Process for lap joining two kinds of metallic members having different melting points

ABSTRACT

A circular blank made of a steel plate is lapped onto an Al-based plate in lapped areas of the Al alloy plate and another steel plate. Then, the three members are pressed by a pair of electrodes, and a welding current is allowed to flow between both the electrodes, thereby sequentially performing the melting of a current-supplied portion of the Al alloy plate and a portion near the current-supplied portion, the elimination of a molten portion by a partial bulgy deformation of the blank toward the Al alloy plate, the abutment of the bulgy deformed portion against the steel plate, and the resistance-welding between the bulgy deformed portion and the steel plate.

FIELD OF THE INVENTION

The present invention relates to a process for lap-bonding of two metalmembers having different melting points, and particularly, to such a lapbonding process which includes lapping a first metal member and a secondmetal member having a melting point higher than that of the first metalmember onto each other, and bonding the lapped areas to each other.

BACKGROUND ART

If a spot welding process using a large electric current is utilized tobond lapped areas of two metal members having different melting points,e.g., an Al-based member (aluminum having a melting point of 660° C.)and an Fe-based member (iron having a melting point of 1,540° C.), anugget is formed on the Al-based member following melting of the latterdue to a difference in melting points between both the members, but aphenomenon occurs that the Fe-based member is hardly molten.

If the strength of weld zone of such different members is examined, itcan be seen that the weld zone shows a strength substantially equal tothat of the weld zone of Al-based members, namely the same type ofmembers in a tensile shearing test, but shows a strength, for example,of only about one sixth of that of the same type of the members in aU-tensile test.

Therefore, it is a conventional practice to employ a process in which aclad material comprised of an Al alloy layer and a steel layer isinterposed between the lapped areas of the Al-based member and theFe-based member, with the Al alloy layer located on the side of Al-basedmember and the steel layer located on the side of Fe-based member (seeJapanese Patent Application Laid-open No.111778/1993).

However, the prior art process suffers the following problems: In a casewhere the lapped areas have a complicated shape such as an arcuateshape, the accommodatability is poor, and a gap is produced between theAl-based member and the Fe-based member in the lapped areas dependingupon the thickness of the clad material and as a result, the placeswhere this process can be applied are largely limited in respect of thedesign. In a case where the clad materials are dotted between the lappedareas, the air-tightness of the weld zone is injured by the gap. On theother hand, in a case where the clad material is mounted over the entirelength of the lapped areas, an increase in weight is caused. Inaddition, the clad material is relatively expensive and hence, anincrease in manufacture cost of the bonded product cannot be avoided.

A further attempt has been made to provide a solid-phase bonding betweenFe-based and Al-based members by decreasing a welding current.

For example, Japanese Patent Application Laid-Open No. 7-214338discloses a technique for bonding an Fe-based metal material and anAl-based metal material by a resistance welding with use of a pin madeof an Fe-based metal material having a substantially T-shaped section.However, in the case of this prior art process, the pin which is pressedby an electrode to penetrate through at least one of the materials has acomplicated shape. For this reason, there are problems that themanufacture cost for the pin is increased, and in the bonding operation,labors are required by positioning and handling of the pin, resulting ina poor efficiency.

Further, the surface of the Al-based member is covered with a firm oxidefilm and for this reason, an enhancement in a bond strength to beprovided by the solid phase bonding is hindered by the oxide film.

To avoid this, it is necessary to subject the Al-based member to anoxide film removing treatment, e.g., a brushing using a wire brush.However, the carrying-out of such a treatment is undesirable, because itincreases the operating steps and the operating cost.

Furthermore, Japanese Patent Publication No. 52-2378 teaches a techniquefor bonding materials by a spot welding, which comprises preparing ahard material having a relatively large hardness and a high meltingpoint and a soft material having a relatively small hardness and a lowmelting point, forming at least one of the materials into a roundedbar-like shape, and lapping the materials onto each other to bond themto each other, while pressing them from above and below by the pair ofelectrodes. With this process, an oxide film generated in the surface ofthe soft material formed by an Al alloy, for example, can be destroyedby a plastic deformation, and therefore, there is an advantage ofenhancing the welding strength. In addition, a recessed groovepositioning the hard material in a predetermined position is provided inone of the electrodes pressing the hard material, and therefore, thereis an advantage that any deviation in the relative positionalrelationship between both the materials can effectively be prevented.However, the hard material opposed to the electrode in which therecessed groove is formed is limited to ones having such a shape thatcan be engaged into the recessed groove, and therefore, there is ademerit that the utilization is limited.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide abonding process of the above-described type, wherein even in a casewhere lapped areas have a complicated shape, the accommodatability isgood, and the generation of a gap in the lapped areas can be avoided andthe workability is enhanced and moreover, the manufacture cost of thebonded product can be reduced.

To achieve the above object, according to the present invention, thereis provided a lap bonding process which includes lapping a first metalmember and a second metal member having a melting point higher than thatof the first metal member onto each other, and bonding resulting lappedareas to each other, the process comprising the steps of lapping a plateshaped third metal member onto the first metal member in the lappedareas, the third metal member having a melting point higher than that ofthe first metal member and being capable of being plastically deformedand resistance-welded to the second metal member; pressing the first,second and third metal members by a pair of electrodes and allowing awelding current to flow between both the electrodes, therebysequentially performing a melting of a current-supplied portion of thefirst metal member and a portion of the first metal member near thecurrent-supplied portion, an elimination of a molten portion produced inthe first metal member by a partial bulgy deformation of at least one ofthe second and third metal members, and a resistance welding of thesecond and third metal members through a bulgy deformed portion of theat least one member.

With the above process, the first and second metal members are firmlybonded to each other through a bulgy deformed portion.

The plate-shaped third metal member may be a blank produced by punchingand hence, has a large degree of freedom in the shape. As a result, evenwhen the lapped area has a complicated shape, It is possible to easilyaccommodate the complicated shape.

Further, the plate-shaped third metal member is lapped onto the firstmetal member in the lapped area and hence, a gap cannot be producedbetween the first and second metal members.

Moreover, a plate-shape member is used as the third metal member andhence, increases in manufacture cost and weight of a bonded product dueto use of the third metal member are suppressed.

It is another object of the present invention to provide a bondingprocess of the above-described type, wherein in the course of weldingthe Al-based member to the Fe-based member, various shapes of themembers are applicable.

To achieve the above object, according to the present invention, thereis provided a lap bonding process for lapping a first metal member and asecond metal member having a melting point higher than that of the firstmetal member onto each other, and bonding resulting lapped areas to eachother, the process including the steps of selecting an Al-based memberhaving a planar portion as the first metal member and an Fe-based memberhaving a planar portion as the second metal member; lapping the firstand second metal members onto each other at the planar portions;pressing the lapped areas by a pair of electrodes and supplying anelectric current between the electrodes, thereby forming a recess on abonded surface of the Al-based member by a deformation of the Al-basedmember through medium of a pressed and current-supplied portion of theFe-based member; and bonding the pressed and current-supplied portionand the Al-based member at the recess.

In the above process, by lapping the first and second metal members ontoeach other at the planar portions, and pressing and supplying a currentto the lapped areas, a recessed portion is formed in the Al-based firstmetal member due to its deformation. And the welding is performedutilizing the recessed portion, thereby enabling the process to beapplied in infinitely wide fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a first example of a bonded product;

FIG. 2 is a sectional view taken along a line 2--2 in FIG. 1;

FIG. 3 is a view for explaining a spot welding machine;

FIG. 4 is a perspective view of an essential portion of a first exampleof an upper electrode;

FIG. 5 is a perspective view of an essential portion, showing oneexample of a state in which a circular blank, an aluminum alloy plateand a steel plate have been lapped together;

FIG. 6 is an explanatory view showing a state in which the circularblank, the aluminum alloy plate and the steel plate are pressed betweenupper and lower electrodes and a welding current is allowed to flowbetween both the electrodes;

FIG. 7 is an explanatory view showing a state in which a portion of thealuminum alloy plate has been molten;

FIG. 8 is an explanatory view showing a state in which the circularblank has been deformed to be partially bulgy;

FIG. 9 is an explanatory view showing a state in which the bulgydeformed portion and the steel plate have been bonded to each other;

FIG. 10 is a perspective view of the circular blank having an adhesiveapplied thereto;

FIG. 11 is a sectional view showing another example of the state inwhich the circular blank, the aluminum alloy plate and the steel platehave been lapped together;

FIG. 12A is a perspective view of an essential portion, showing a secondexample of the bonded product;

FIG. 12B is a sectional view taken along a line 12B--12B in FIG. 12A;

FIG. 13 is a perspective view of an essential portion, showing a thirdexample of the bonded product;

FIG. 14 is a perspective view of an essential portion, showing a fourthexample of the bonded product;

FIG. 15 is an exploded perspective view of a U-tensile test piece;

FIG. 16 is a perspective view showing a first example of a U-tensiletest piece;

FIG. 17 is a perspective view showing a second example of the U-tensiletest piece;

FIG. 18 is a perspective view showing a third example of the U-tensiletest piece;

FIG. 19A is a photomicrograph showing the metallographic structure on asection of a bonded portion;

FIG. 19B is a reduced tracing of the photomicrograph shown in FIG. 19A;

FIG. 20 is a front view of an essential portion of a second example ofthe upper electrode;

FIG. 21 is a view taken along a line 21--21 in FIG. 20;

FIG. 22 is a perspective view of an essential portion of a third exampleof the upper electrode;

FIG. 23 is a perspective view of an essential portion of a fourthexample of the upper electrode;

FIG. 24 is a plan view of a fifth example of the bonded product;

FIG. 25 is a sectional view taken along a line 25--25 in FIG. 24,showing the relationship between the bonded product and the upper andlower electrodes;

FIG. 26 is a sectional view showing a state in which a steel plate andan aluminum alloy plate are pressed between upper and lower electrodesand a welding current is allowed to flow between both the electrodes;

FIG. 27 is a sectional view showing a state in which a portion of thealuminum alloy plate has been molten;

FIG. 28 is a sectional view showing a state in which the steel plate andthe aluminum alloy plate have been bonded to each other;

FIG. 29 is a sectional view showing the relationship of the steel plate,the aluminum alloy plate and an Ni insert lapped together;

FIG. 30 is a sectional view showing a state in which the steel plate andan aluminum-based hollow extrudate have been bonded to each other;

FIG. 31 is a sectional view of an aluminum-based solid extrudate; and

FIG. 32 is a front view of a U-tensile test piece.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVEVTION EXAMPLE I

Referring to FIGS. 1 and 2, a bonded product 1 includes an Al alloyplate (or an Al plate) 2 as an Al-based member which is a first metalmember, and a steel plate (an Fe alloy plate or an Fe plate) 3 as anFe-based member which is a second metal member having a melting pointhigher than that of the Al alloy plate 2, with lapped areas 4 of theplates 2 and 3 being bonded to each other.

For the lap bonding process, a circular blank 5 made by punching from athird metal member having a melting point higher than that of the Alalloy plate 2, e.g., a steel plate, is used, and a spot welding as aresistance welding is utilized.

A bonded structure produced by the lap bonding process is such that abulgy deformed portion 8 resulting from that plastic deformation of acentral portion of the circular blank 5 which has been produced bypressing the members by upper and lower electrodes 6 and 7 made by O. F.C. and by supplying a welding current is spot-welded to the steel plate3 to form a nugget 9, with a molten portion of the Al alloy plate 2being eliminated, and an outer peripheral portion 10 of the circularblank 5 is in pressure contact with the Al alloy plate 2.

The lap bonding process will now be described in detail.

Referring to FIGS. 3 and 4, an upper electrode 6 of a spot weldingmachine 11 is comprised of a rod-like electrode body 12 which iscircular in section, and a truncated conical protrusion 14 provided on alower end face of the electrode body 12 to project therefrom and havinga draft 13. Therefore, the protrusion 14 has a circular section within aplane intersecting an axial direction of the electrode. A roundedportion 16 is provided at a peripheral edge of a smaller end face 15 ofthe protrusion 14. A JIS R-type electrode is used as a lower electrode7, but a JIS CF-type electrode or a CR-type electrode may also be used.In Figures, reference numeral 17 is a transformer, and 18 is aninverter-type controller.

(a) As shown in FIG. 5, one end of the Al alloy plate 2 is lapped ontoone end of the steel plate 3 and then, the circular blank 5 is lappedonto the Al alloy plate 2 in the lapped area 4.

(b) As shown in FIG. 6, the circular blank 5, the Al alloy plate 2 andthe steel plate 3 are disposed between both the electrodes 6 and 7 withthe circular blank 5 located on the side of the upper electrode 6, andthen, those members 5, 2 and 3 are pressed by both the electrodes 6 and7, and at the same time, a welding current is allowed to flow betweenboth the electrodes 6 and 7.

(c) As shown in FIG. 7, the circular blank 5, the Al alloy plate 2 andthe steel plate 3 are heated by a contact resistance as a result ofsupplying of the current in the state in which they have been pressed,and then, the current-supplied portion 19 of the Al alloy plate 2 havinga lower melting point and a portion near the current-supplied portion 19are molten, while the current-supplied portions of the circular blank 5and the steel plate 3 and portions near them are softened.

(d) As shown in FIG. 8, the central portion of the circular blank 5pressed by the truncated conical protrusion 14 of the upper electrode 6is deformed to be bulgy toward the Al alloy plate 2 to form a truncatedconical shape, whereby the molten portion is eliminated and moved to agap between the Al alloy plate 2 and the steel plate 3. Therefore, asmaller end 20 of the bulgy deformed portion 8 is put into abutmentagainst the steel plate 3.

(e) As shown in FIG. 9, the smaller end 20 of the bulgy deformed portion8 and the steel plate 3 abutting against the smaller end 20 are suppliedwith the current in the state in which they are by the electrodes 6 and7. Therefore, the smaller end 20 and the steel plate 3 are spot-weldedto each other to form the nugget 9, thereby forming the welded zone inthe same-type materials.

After such spot-welding, the truncated conical protrusion 14 of theupper electrode 6 is easily withdrawn from the bulgy deformed portion 8,because it has the draft 13.

With the above-described process, the Al alloy plate 2 is firmly bondedto the steel plate 3 with a rivet coupling-like fastened structureprovided by the outer peripheral portion 10 and the bulgy deformedportion 8 of the circular blank 5.

In addition, the circular blank 5 made by punching has a larger degreeof freedom in the shape and as a result, even when the lapped area 4 hasa complicated shape, it is possible to easily accommodate this.

Further, the circular blank 5 is lapped onto the Al alloy plate 2 in thelapped area 4 and hence, a gap cannot be produced between the Al alloyplate 2 and the steel plate 3.

Moreover, the circular blank 5 is of a single-plate structure and hence,increases in manufacture cost and weight of the bonded product 1 due tothe use of the circular blank 5 are inhibited.

In the above-described lap bonding example, the bulgy deformed portion 8can be formed on the steel plate 3 using the lower electrode 7 havingthe same shape as the upper electrode 6. Alternatively, bulgy deformedportions 8 can be formed on both of the circular blank 5 and the steelplate 3, respectively.

As shown in FIG. 10, an adhesive 22 is applied to the entire peripheryof that surface 21 of the circular blank 5 which is opposed to the Alalloy plate 2, whereby the circular blank 5 can be reliably retained ata predetermined position in the lapped area 4 to enhance the bondingoperability.

As shown in FIG. 11, the adhesive 22 may be replaced by a sealingmaterial 23, and the sealing material 23 may be interposed in an annulusbetween the Al alloy plate 2 and the steel plate 3 in the lapped area 4,whereby the corrosion resistance of the bonded zone including a bore 24(see FIG. 9) produced in the Al alloy plate 2 by the bonding operationcan be enhanced.

FIGS. 12 to 14 show three examples of bonded products 1, wherein a thirdmetal member is formed utilizing the plastically deforming ability ofthe steel plate 3.

The example shown in FIGS. 12A and 12B was produced in the followingmanner: One end 25 of the steel plate 3 was folded back. The folded-backportion 5₁ is used as a third metal member. Then, one end of the Alalloy plate 2 was inserted between the steel plate 3 and the folded-backportion 5₁, so that both the planes of both the Al alloy plate 2 and thesteel plate 3 were parallel to each other, and the directions ofextensions of the plates 2 and 3 crossed each other at 90°. Thereafter,a bonding process similar to that described above was carried out.

The example shown in FIG. 13 was produced in the following manner: Aplate-like protrusion 26 was provided at one side edge of an end of thesteel plate 3, so that the plate-like protrusion 26 and the steel plate3 are located on the same plane. The plate-like protrusion 26 was foldedback, and the folded-back portion 5₁ was used as a third metal member.Then, one end of the Al alloy plate 2 was inserted between the steelplate 3 and the folded-back portion 5₁, so that both the planes of theAl alloy plate 2 and the steel plate 3 were parallel to each other andthe plates 2 and 3 extended in the same direction. Thereafter, a bondingprocess similar to that described above was carried out.

The example shown in FIG. 14 was produced in the following manner:Substantially half of a plate-like folded portion 27 formed by foldingone end of the steel plate 3 at right angle was folded back, and thefolded-back portion 5₁ was used as a third metal member. Then, one endof the Al alloy plate 2 was inserted between the steel plate 3 and thefolded-back portion 5₁, so that both the planes of the Al alloy plate 2and the steel plate 3 were in a right angle relation to each other andthe directions of extensions of the plates 2 and 3 crossed each other at90°. Thereafter, a bonding process similar to that described above wascarried out.

Particular examples will be described below.

A. U-tensile Strength

As shown in FIG. 15, first halves 28 for a plurality of U-tensile testpieces were made from the Al alloy plate 2, and second halves 29 for aplurality of U-tensile test pieces were made from the steel plate 3,both according to JIS Z 3137. Further, the steel plate 3 was subjectedto a punching to provide a plurality of circular blanks 5.

The material for the Al alloy plate 2 is JIS 5182 and had a thickness t₁set at 1.0 mm. On the other hand, the material for the steel plate 3 isJIS SPCC and had a thickness t₂ set at 0.7 mm. In this case, t₁=(2^(1/2))×t₂ is established between the thickness t₁ of the Al alloyplate 2 and the thickness t₂ of the steel plate 3. This is for thepurpose of ensuring that the plates 2 and 3 have substantially the samerigidity. The circular blank 5 had a diameter D₁ set at 15 mm.

As shown in FIG. 3, in the upper electrode 6, the diameter D₂ of theelectrode body 12 is set at 16 mm; the taper angle θ of the truncatedconical protrusion 14 is set at 90 degrees; the length L is set 4 mm;and the radius R₁ of the rounded portion 16 at the peripheral edge ofthe smaller end face 15 is set at 1 mm. The smaller-end diameter D₃ isvaried in a range of 4 to 7 mm.

In the lower electrode 7, the diameter D₄ is set at 16 mm; and theradius R₂ of a spherical tip end face 30 is set at 80 mm.

A plurality of U-tensile test pieces 31 as shown in FIG. 16 according toan embodiment were produced by carrying out a bonding process similar tothat described above (see FIGS. 5 to 9), except that the first andsecond halves 28 and 29 and a circular blank 5 were used and the weldingconditions and the upper electrode 6 were changed.

Then, a U-tensile test piece 32 shown in FIG. 17 according to acomparative example 1 was produced by carrying out a spot welding usingthe first and second halves 28 and 29 and using two lower electrodes 7as upper and lower electrodes, respectively.

Further, a U-tensile test piece 33 shown in FIG. 18 according to acomparative example 2 was produced by carrying out a spot welding usingthe two first halves 28 and using two lower electrodes 7 as upper andlower electrodes, respectively.

Thereafter, the U-tensile test pieces 31 to 33 were subjected to atensile test.

Table 1 shows the smaller-end diameter D₃ of the upper electrode 6, thewelding conditions, the amount of expulsion and surface flash and theU-tensile strength for the U-tensile test pieces 31 to 33.

                                      TABLE 1                                     __________________________________________________________________________    Smaller-                                                                        end Welding conditions                                                      diameter D.sub.3 Press-                                                                             Current                                                   (mm) of Welding ing supplying Amount of U-tensile                             upper current force time expulsion and strength                               electrode (kA) (kgf) (cycle) surface flash (kgf)                            __________________________________________________________________________    Example 1                                                                           4     10   200  20   smaller                                                                              105                                           Example 2 5 10 200 20 smaller 130                                             Example 3 6 12 200 20 slightly 150                                                 larger                                                                   Example 4 7 14 200 20 larger 200                                              Comparative -- 16 200 4 smaller  15                                           example 1                                                                     Comparative -- 24 400 4 smaller  95                                           example 2                                                                   __________________________________________________________________________

As apparent from Table 1, it can be seen that the U-tensile strength ofthe test pieces according to Examples 1 to 4 is largely enhanced andexceeds the strength of bonding of the Al alloy plates according to thecomparative example 2. It can be seen that the U-tensile strength of thetest piece according to comparative example 2 is approximately one sixthof that of the comparative example 1.

FIG. 19A is a photomicrograph showing the metallographic structure of asection of the bonded zone of the test piece which is Example 1, andFIG. 19B is a reduced tracing of the photomicrograph shown in FIG. 19A.It can be seen from FIGS. 19A and 19B that the nugget 9 was formedbetween the smaller end 20 of the bulgy deformed portion 8 and thesecond half 29, whereby the first and second halves 28 and 29 werefirmly bonded to each other.

If the diameter D₃ of the smaller end of the upper electrode 6 is equalto or larger than 6 mm as in Examples 3 and 4, the U-tensile strength ishigher, but an expulsion and surface flash is generated.

Then, U-tensile test pieces 31 to 33 similar to those described abovewere produced by carrying out a bonding process similar to thatdescribed above, except that the thickness t₁ of the first half 28 waschanged to 1.2 mm; the thickness t₂ of the second half 29 was changed to0.8 mm and further, the welding conditions were partially changed.

Table 2 shows the smaller end diameter D₃ of the upper electrode 6, thewelding conditions, the amount of expulsion and surface flash and theU-tensile strength for the U-tensile test pieces 31 to 33.

                                      TABLE 2                                     __________________________________________________________________________    Smaller-                                                                        end Welding conditions                                                      diameter D.sub.3 Press-                                                                             Current                                                   (mm) of Welding ing supplying Amount of U-tensile                             upper current force time expulsion and strength                               electrode (kA) (kgf) (cycle) surface flash (kgf)                            __________________________________________________________________________    Example 5                                                                           4     10   200   4   smaller                                                                              181                                           Example 6 5 12 200 20 smaller 205                                             Example 7 6 14 200 20 slightly 240                                                 larger                                                                   Example 8 7 14 200 20 larger 260                                              Comparative -- 16 400  4 smaller 20                                           example 3                                                                     Comparative -- 24 200  4 smaller 180                                          example 4                                                                   __________________________________________________________________________

It can be seen that a tendency similar to that in Table 1 is recognizedeven in the case of Table 2.

B. Taper Angle θ of Truncated Conical Protrusion of Upper Electrode

A plurality of upper electrodes 6 each having a changed taper angle θ ofa truncated conical protrusion 14 thereof were prepared. In this case,in the upper electrode 6, the diameter D₂ of the electrode body 12 wasset at 16 mm; the smaller-end diameter D₃ of the truncated conicalprotrusion 14 was set at 4 mm; the length L of the truncated conicalprotrusion 14 was set at 3 mm; and the radius R₁ of the rounded portion16 of the peripheral edge of the smaller end 15 was set at 1 mm.

In the lower electrode 7, the diameter D₄ was set at 16 mm, and theradius R₂ of the spherical tip end 30 was set at 80 mm.

A plurality of Al alloy plates 2, a plurality of steel plates 3 and aplurality of circular blanks 5 each made by punching of a steel plate ofthe same type as of the steel plates 3 were also prepared. The materialfor the Al alloy plate 2 was JIS 5182 and had a thickness t₁ set at 1.0mm. The material for the steel plate 3 was JIS SPCC and had a thicknesst₂ set at 0.7 mm. The diameter D₁ of the circular blank 5 was set at 15mm.

Then, a bonding process similar to that described above (see FIGS. 5 to9) was carried out to find the relationship between the taper angle θand the mold release failure rate P, thereby giving a result shown inTable 3.

The welding conditions were as follows: The welding current was 10 kA;the pressing force was 200 kgf; and the current supplying time was 20cycles. The mold release failure rate P was determined according to anequation, P=(n/10)×100 (%), wherein the number of runs of a bondingoperation carried out using the upper electrode 6 provided with thetruncated conical protrusion 14 having a predetermined taper angle θ was10; and the frequency of adhesion of the truncated conical protrusion 14to the inner surface of the bulgy deformed portion 8 was represented byn. The term "adhesion" means a mechanically fitted state to the extentwhich permits the truncated conical protrusion 14 to be removed from theinner surface of the bulgy deformed portion 8 by striking the bondedproduct 1 by a hammer.

                  TABLE 3                                                         ______________________________________                                                      Taper angle θ (degree)                                                    0       30    60    90  120                                   ______________________________________                                        Mode release failure rate P (%)                                                               100     90    80    40  0                                     ______________________________________                                    

As apparent from Table 3, the mold release failure rate P can beremarkably reduced by setting the taper angle θ in a range of θ≧90degree.

C. FIGS. 20 and 21 show a modification to the upper electrode 6. Theupper electrode 6 is comprised of a rod-like electrode body 12 which iscircular in section, and a columnar protrusion 14₁ projectingly providedon a lower end face of the electrode body 12. Therefore, the protrusion14₁ is a straight protrusion and has a circular section in a plane whichintersects the direction of an electrode axis. The protrusion 14₁ servesto form a bulgy deformed portion 8 on a circular blank 5, and has arounded portion 16₁ provided at an edge of a tip end face 15₁, i.e., ata peripheral edge.

To determine the relationship between the radius R₃ of the roundedportion 16₁ and the mold release failure rate P, a plurality of upperelectrodes 6 having different radii R₃ were prepared. In each of theupper electrodes 6, however, the diameter D₂ of the electrode body 12was set at 16 mm; the length L of the protrusion 14₁ was set at 5 mm;and the diameter D₃ of the tip end face 15₁ was set at 4 mm.

As shown in FIG. 3, in a lower electrode 7, the diameter D₄ was set at16 mm, and the radius R₂ of the spherical tip end face 30 was set at 80mm.

A plurality of Al alloy plates 2, a plurality of steel plates 3, and aplurality of circular blanks 5 made by punching from a steel plate ofthe same type as the steel plates 3 were also prepared. The material forthe Al alloy plate 2 was JIS 5182 and had a thickness t₁ set at 1.0 mm.The material for the steel plate 3 was JIS SPCC and had a thickness t₂set at 0.7 mm. The diameter D₁ of the circular blank 5 was set at 15 mm.

Then, a bonding process similar to that described above (see FIGS. 5 to9) was carried out to find the relationship between the radius R₃ andthe mold release failure rate P, thereby giving a result shown in Table4.

The welding conditions were as follows: The welding current was 10 kA;the pressing force was 200 kgf; and the current supplying time was 20cycles.

                  TABLE 4                                                         ______________________________________                                                      Radius R.sub.3 (mm) of rounded portion                                          0        1       2      3                                     ______________________________________                                        Mold release failure rate P (%)                                                               100      100     20     0                                     ______________________________________                                    

As apparent from Table 4, the mold release failure rate P can beremarkably reduced by setting the radius R₃ of the rounded portion 16₁in a range of R₃ ≧2 mm.

The protrusion in the upper electrode 6 may have a non-circular section,e.g., a square section as shown in FIGS. 22 and 23, without having acircular section as described above in the plane intersecting thedirection of the electrode axis. Namely, the protrusion 14₂ shown inFIG. 22 assumes a truncated quadrangular pyramidal shape and has a draft13. The protrusion 14₃ shown in FIG. 23 assumes a quadrangular columnarshape and has a rounded portion 16₁ provided at an edge of the tip endface 15₁, i.e., at a peripheral edge.

If the upper electrode 6 is constructed in the above manner, therelative rotation between the Al alloy plate 2 and the steel plate 3 canbe reliably prevented.

Referring to FIGS. 24 and 25, a bonded product 1 includes an Al alloyplate 2 and a steel plate 3, lapped areas 4 of which are bonded byutilizing a spot welding process as a resistance welding process, usinga pair of upper and lower electrodes 6 and 7. In the bonded structure, asubstantially truncated conical pressed/current supplied portion 40bulged from the steel plate 3 and a substantially truncated conicalrecess 41 in the Al alloy plate 2 are in a fitted relation to eachother, and a solid phase bonding is produced between the pressed/currentsupplied portion 40 and the Al alloy plate 2 at the recess 41. Namely,the plates 2 and 3 are bonded by a diffusion phenomenon in a very smallarea of a bond interface. In this case, no nugget is generated, or evenif a nugget is generated, it is extremely small and hence, littlecontribute to the bonding.

The spot welding between the Al alloy plate 2 and the steel plate 3 willbe described.

In FIGS. 4 and 25, an inverter welding machine is used as a spot weldingmachine, and includes an upper electrode 6 which is comprised of arod-like electrode body 12 which is circular in section, and a truncatedconical protrusion 14 projectingly provided on a lower end face of theelectrode body 12 and having a draft 13. The protrusion 14 has a roundedportion 16 provided at a peripheral edge of a smaller end face 15. Anelectrode of JIS R type is used as a lower electrode 7, but an electrodeof JIS CF type or CR type may be used.

(a) As shown in FIG. 26, one end of a steel plate 3 is lapped onto oneend of an Al alloy plate 2. Then, lapped areas 4 are disposed betweenboth the electrodes 6 and 7 with the steel plate 3 located on the sideof the upper electrode 6, and are then pressed by both the electrodes 6and 7, while a welding current is allowed to flow between both theelectrodes 6 and 7.

(b) As shown in FIG. 27, by supplying of the current in the pressedstate, pressed and current-supplied portions 40 and 42 of the steelplate 3 and the Al alloy plate 2 are softened, while at the same time,the bonded surface 43 of the pressed and current-supplied portion 42 ofthe Al alloy plate 2 having a lower melting point is slightly molten toform a small molten pool 44.

(c) As shown in FIG. 28, the pressing force of the truncated conicalprotrusion 14 of the upper electrode 6 ensures that the pressed andcurrent-supplied portion 40 of the steel plate 3 is bulged into asubstantially truncated conical shape toward the Al alloy plate 2 by theplastic deformation of the steel plate 3, and a substantially truncatedconical recess 41 by the plastic deformation of the Al alloy plate 2 isdefined in the bonded surface 43 of the Al alloy plate 2 by the pressedand current-supplied portion 40. The molten metal in the molten pool 44including an oxide film is discharged into a gap between both the plates2 and 3 during the recess 41 is defined.

Since the recess 41 is defined by the melting of a portion of the Alalloy plate 2 and by the plastic deformation in the above manner, acleaned surface is exposed in an area where the molten pool 44 hasexisted, as a result of the discharging of the molten metal, and acleaned surf ace is exposed around the area where the molten pool 44 hasexisted, by the division of the oxide film by the plastic deformation ofthe Al alloy plate 2.

Thus, a firm solid-phase bonding Is produced between these cleanedsurfaces and the pressed and current-supplied portion 40 of the steelplate 3.

After the above-described spot welding, the truncated conical protrusion14 of the upper electrode 6 is easily withdrawn from the pressed andcurrent-supplied portion 40, because it has the draft 13.

The formation of the molten pool 44 is not an essential requirement.Even if the molten pool 44 is not formed, the division of the oxide filmis performed by the plastic deformation of the Al alloy plate 2 andhence, the cleaned surface is exposed in the recess 41.

As shown in FIG. 29, if a foil-like Ni-insert 45 made of only nickel isdisposed between the steel plate 3 and the Al alloy plate 2 in thelapped areas 4, the bond strength can be enhanced more than in a casewhere the steel plate 3 and the Al alloy plate 2 are bonded directly toeach other in a solid phase manner. This is because the strength ofsolid-phase bonding between the steel plate 3 and the Ni-insert 45 aswell as between the Ni-insert 45 and the Al alloy plate 2 is higher thanthe strength of solid-phase bonding between the steel plate 3 and the Alalloy plate 2. Another reason is that nickel has an effect of breakingthe oxide film on the surface of the Al alloy plate 2.

The Ni-insert 45 may be formed on the steel plate 3 or the Al alloyplate 2 by a plating process. Alternatively, the Ni-insert 45 may beformed on a steel foil or an Al alloy foil by a plating process. In theformer case, the steel foil is opposed to the steel plate 3, and in thelatter case, the Al alloy foil is opposed to the Al alloy plate 2.

The Al-based member is not limited to the Al alloy plate 2, and a hollowextrudate 46 quadrilateral in cross section or a band-like solidextrudate 47 may be used, as shown in FIGS. 30 and 31. The Fe-basedmember is not limited to the steel plate 3, and an angle material or thelike may be used.

EXAMPLE 1

As shown by a dashed line in FIG. 32, a plurality of first halves 28 forU-tensile test pieces were fabricated from an Al alloy plate 2, and aplurality of second halves 29 for U-tensile test pieces were fabricatedfrom a steel plate 3, according to JIS Z 3137. The material for the Alalloy plate 2 was JIS 5182, and the thickness t₁ of the Al alloy plate 2was set at 1 mm. On the other hand, the material for the steel plate 3was JIS SPCC, and the thickness t₂ of the steel plate 3 was set at 0.7mm.

As best shown in FIG. 25, in the upper electrode 6, the diameter D₂ ofthe electrode body 12 was set at 16 mm; the taper angle θ of thetruncated conical protrusion 14 was set at 90 degrees; the length L ofthe truncated conical protrusion 14 was set at 4 mm; and the radius R₁of the rounded portion 16 at the peripheral edge of the smaller end face15 was set at 1 mm. The smaller-end diameter D₃ was varied in a range of3 to 5 mm.

In the lower electrode 7, the diameter D₄ thereof was set at 16 mm, andthe radius R₂ of the spherical tip end face 30 was set at 80 mm.

Using the first and second halves 28 and 29, examples 1 to 3 ofU-tensile test pieces 31 according to the embodiment as shown by solidlines in FIG. 32 were produced by carrying out the same process as shownin FIGS. 26 and 28, except that the welding conditions were setuniformly, and the upper electrode 6 was changed.

Then, using the first and second halves 28 and 29, examples 4 and 5 ofU-tensile test pieces 31 were produced as comparative examples bycarrying out the same spot welding, except that two lower electrodes 7were used as upper and lower electrodes, and the welding conditions werevaried. Thereafter, the examples 1 to 5 were subjected to a tensiletest.

Table 5 shows the smaller-end diameter D₃ of the upper electrode 6, thewelding conditions, the amount of expulsion and surface flash and theU-tensile strength for the examples 1 to 5.

                                      TABLE 5                                     __________________________________________________________________________         Smaller-end                                                                         Welding conditions                                                 U-   diameter D.sub.3                                                                             Current                                                     tensile (mm) of Welding Pressing supplying Amount of U-tensile                test upper current force time expulsion and strength                          piece electrode (kA) (kgf) (cycle) surface flash (kgf)                      __________________________________________________________________________    Example 1                                                                          3     10   200 10   smaller                                                                              55                                              Example 2 4    slightly 50                                                         larger                                                                   Example 3 5    larger 52                                                      Example 4 -- 16 200  4 smaller 15                                             Example 5  10  10  10                                                       __________________________________________________________________________

As apparent from Table 5, examples 1 to 3 according to the embodimenthave a high U-tensile strength, because the solid-phase bonding wasproduced between both the halves 28 and 29 by the cleaned surface of thefirst half 28.

In the case of example 4 as a comparative example, a nugget is formed onthe first half 28, because the welding current is raised more than thatof examples such as example 1. As a result, the U-tensile strength issignificantly reduced, as compared with example 1 or the like.

In the case of example 5 as a comparative example, the solid-phasebonding was produced between both the halves 28 and 29, because thewelding conditions were set in the same manner as in example 1 or thelike. However, such solid-phase bonding was produced mainly between theoxide film of the first half 28 and the second half 29 and hence, theU-tensile strength is significantly reduced, as compared with example 1or the like.

It should be noted that the U-tensile strength of a U-tensile test piecemade through a spot welding process using the two first halves 28 andthe two lower electrodes 7 as upper and lower electrodes was 95 kgf. Inthis case, the welding current was set at 24 kA; the pressing force wasset at 400 kgf; and the current supplying time was set at 10 cycles, andboth the halves 28 were bonded to each other through a nugget formedover both the halves.

EXAMPLE 2

Using first and second halves 28 and 29, an upper electrode 6 having asmaller-end diameter D₃ of 3 mm and an Ni-insert 45 which are similar tothose in EXAMPLE-1, examples 1 to 3 of U-tensile test pieces 31according to the embodiment as shown by solid lines in FIG. 32 wereproduced by carrying out the same process as shown in FIGS. 26 to 29.

Then, using first and second halves 28 and 29 and an Ni-insert 45 whichare similar to those in EXAMPLE-1 and using two lower electrodes 7 asupper and lower electrodes, respectively, a spot welding process wascarried out with welding conditions set uniformly, thereby producingexample 4 of the U-tensile test piece 31 as a comparative example.Thereafter, examples 1 to 4 were subjected to a tensile test.

Table 6 shows the construction of the Ni-insert 45, the smaller-enddiameter D₃ of the upper electrode 6, the welding conditions, the amountof expulsion and surface flash and the U-tensile strength for examples 1to 4.

                                      TABLE 6                                     __________________________________________________________________________                  Smaller-end                                                                         Welding conditions                                                                          Amount of                                                 diameter D.sub.3                                                                             Current                                                                            expulsion                                       (mm) of Welding Pressing supplying and U-tensile                            U-tensile Construction of upper current force time surface strength                                                 testpiece Ni-insert electrode                                                (kA) (kgf) (cycle) flash (kgf)         __________________________________________________________________________    Example 1                                                                          Ni foil having a                                                                       3     10   200 10   smaller                                                                            98                                        thickness of                                                                  100 μm                                                                    Example 2 Ni foil having a      100                                            thickness of                                                                  50 μm                                                                     Example 3 Ni-plated layer      87                                              having a thickness                                                            of 20 μm on a steel                                                        foil having a                                                                 thickness of                                                                  100 μm                                                                    Example 4 Ni foil having a -- 10 200 10 smaller 35                             thickness of                                                                  100 μm                                                                  __________________________________________________________________________

As apparent from Table 6, in examples 1 to 3 according to theembodiment, a solid-phase bonding by the cleaned surface of the firsthalf 28 is produced and hence, the U-tensile strength is largelyenhanced to be about 2.5 or more times larger than that of example 4 asthe comparative example. As a result of use of the Ni-insert 45, theU-tensile strength of each of examples 1 to 3 is about 1.6 or more timeslarger than that of example 1 in Table 5 which was produced under thesame conditions as in examples 1 to 3, except that the Ni-insert 45 wasnot used.

EXAMPLE 3

A plurality of hollow extrudates 46 and a plurality of solid extrudates47 shown in FIGS. 30 and 31 were prepared. The material for both theextrudates 46 and 47 is JIS 6063. The size of the hollow extrudates 46is 30 mm in longitudinal length L₁ ; 70 mm in lateral length L₂ and 4 mmin thickness t₃, as shown in FIG. 30. The size of the solid extrudates47 is 60 mm in width W, and 5 mm in thickness t₄, as shown in FIG. 8(b).

Using a second half 29, an upper electrode 6 having a smaller-enddiameter D₃ of 4 mm, extrudates 46 and 47 and an Ni-insert 45 which aresimilar to those in EXAMPLE-1, examples 1 and 2 of U-tensile test piecesaccording to the embodiment were produced by carrying out the sameprocess as shown in FIGS. 26 to 29.

Then, using a second half 29 and extrudates 46 and 47 which are similarto those in EXAMPLE-1 and using two lower electrodes 7 as upper endlower electrodes, a spot welding is carried out with the weldingconditions set uniformly, thereby producing examples 3 and 4 ofU-tensile test pieces as comparative examples. Thereafter, examples 1 to4 were subjected to a tensile test.

Table 7 shows constructions of the used extrudates 46 and 47 and theNi-insert 45, the smaller-end diameter D₃ of the upper electrode 6, thewelding conditions, the amount of expulsion and surface flash and theU-tensile strength for examples 1 to 4.

                                      TABLE 7                                     __________________________________________________________________________                   Smaller-                                                            end Welding conditions Amount of                                                        diameter       Current                                                                            expulsion                                                                          U-                                        Construc- D.sub.3 (mm) of Welding Pressing supplying and tensile                                                   U-tensile Used tion of upper                                                 current force time surface                                                    strength                                test piece extrudate Ni-insert electrode (kA) (kgf) (cycle) flash           __________________________________________________________________________                                            (kgf)                                 Example 1                                                                          Hollow                                                                             Ni foil                                                                            4     12   200 10   smaller                                                                            150                                     Example 2 Solid having a      142                                               thickness                                                                     of 100 μm                                                                Example 3 Hollow -- -- 12 200 10 smaller 55                                   Example 4 Solid       47                                                    __________________________________________________________________________

As apparent from Table 7, if example 1 is compared with example 3 andexample 2 is compared with example 4, the solid-phase bonding by thecleaned surfaces of the hollow and solid extrudates 46 and 47 isproduced in examples 1 and 2, and each of examples 1 and 2 has a highU-tensile strength, as compared with examples 3 and 4, because of use ofthe Ni-insert 45.

In this way, according to EXAMPLE-3, not only the Al-based plate butalso the Al-based extrudate and the Fe-based member can be firmly bondedto each other.

It should be noted that in examples 1 and 2, a seam welding process anda projection welding process, in addition to the spot welding process,are included in the resistance welding process. In the projectionwelding process, a projection which is a pressed and current-suppliedportion 40 is formed on an Fe-based member.

What is claimed is:
 1. A process for lap-bonding of two types of metalmembers having different melting points by lapping a first metal memberand a second metal member having a melting point higher than a meltingpoint of said first metal member onto each other, and bonding resultinglapped areas to each other, said process including the steps ofselecting an Al-based member having a planar portion as said first metalmember and an Fe-based member having a planar portion as said secondmetal member; lapping the first and second metal members onto each otherat said planar portions; pressing said lapped areas by a pair ofelectrodes and supplying a current between both the electrodes, therebyforming a recess on a bonded surface of said Al-based member by adeformation of the Al-based member through medium of a pressed andcurrent-supplied portion of said Fe-based member; and bonding saidpressed and current-supplied portion and said Al-based member to eachother at said recess.
 2. A process for lap-bonding of two types of metalmembers having different melting points by lapping a first metal memberand a second metal member having a melting point higher than a meltingpoint of said first metal member onto each other, and bonding resultinglapped areas to each other, said process including the steps ofselecting an Al-based member having a planar portion as said first metalmember and an Fe-based member having a planar portion as said secondmetal member; lapping the first and second metal members onto each otherat said planar portions; pressing said lapped areas by a pair ofelectrodes and supplying a current between both the electrodes, therebyallowing a pressed and current-supplied portion of said Fe-based memberto be bulged toward said Al-based member by a plastic deformation ofsaid Fe-based member, and at the same time, forming a recess an a bondedsurface of said Al-based member by a deformation of said Al-based memberthrough medium of said pressed and current-supplied portion of saidFe-based member, thereby bonding said pressed and current-suppliedportion and said Al-based member to each other at said recess.
 3. Aprocess for lap-bonding of two types of metal members having differentmelting points according to claim 1 or 2, wherein an Ni-insert isdisposed between said Fe-based member and said Al-based member in saidlapped areas.